1 year before the events of G4, Opaline Arcana was an ordinary white unicorn who worked at a paint factory in Baltimare. One night, while chasing a pair of thieves who broke into the factory, she slipped and fell into a vat of purple paint tainted with lead. She came out with a permanent stained coat…and a manticore’s dose of lead poisoning.
She spent the majority of seasons 1-9 thinking that she was a natural-born alicorn from a kingdom called Skyros and that Celestia and Luna bullied her. By the end of S9, she confronted the Mane Six, delivering a hammy speech about her “backstory” and how Equestria was screwed because Twilight had sealed all of Equestria’s magic into a trio of crystals before laughing maniacally and passing out in front of the group (Pinkie Pie’s response was pressing a button that said, “that was easy”).
Opaline currently resides in a mental hospital in Vanhoover, mumbling something about dragon magic, a fictional seaside town called Maretime Bay, and somepony named Misty.